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<title>FFTW 3.3.8: Guru vector and transform sizes</title>

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<a name="Guru-vector-and-transform-sizes"></a>
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Next: <a href="Guru-Complex-DFTs.html#Guru-Complex-DFTs" accesskey="n" rel="next">Guru Complex DFTs</a>, Previous: <a href="Interleaved-and-split-arrays.html#Interleaved-and-split-arrays" accesskey="p" rel="prev">Interleaved and split arrays</a>, Up: <a href="Guru-Interface.html#Guru-Interface" accesskey="u" rel="up">Guru Interface</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Guru-vector-and-transform-sizes-1"></a>
<h4 class="subsection">4.5.2 Guru vector and transform sizes</h4>

<p>The guru interface introduces one basic new data structure,
<code>fftw_iodim</code>, that is used to specify sizes and strides for
multi-dimensional transforms and vectors:
</p>
<div class="example">
<pre class="example">typedef struct {
     int n;
     int is;
     int os;
} fftw_iodim;
</pre></div>
<a name="index-fftw_005fiodim"></a>

<p>Here, <code>n</code> is the size of the dimension, and <code>is</code> and <code>os</code>
are the strides of that dimension for the input and output arrays.  (The
stride is the separation of consecutive elements along this dimension.)
</p>
<p>The meaning of the stride parameter depends on the type of the array
that the stride refers to.  <em>If the array is interleaved complex,
strides are expressed in units of complex numbers
(<code>fftw_complex</code>).  If the array is split complex or real, strides
are expressed in units of real numbers (<code>double</code>).</em>  This
convention is consistent with the usual pointer arithmetic in the C
language.  An interleaved array is denoted by a pointer <code>p</code> to
<code>fftw_complex</code>, so that <code>p+1</code> points to the next complex
number.  Split arrays are denoted by pointers to <code>double</code>, in
which case pointer arithmetic operates in units of
<code>sizeof(double)</code>.
<a name="index-stride-2"></a>
</p>

<p>The guru planner interfaces all take a (<code>rank</code>, <code>dims[rank]</code>)
pair describing the transform size, and a (<code>howmany_rank</code>,
<code>howmany_dims[howmany_rank]</code>) pair describing the &ldquo;vector&rdquo; size (a
multi-dimensional loop of transforms to perform), where <code>dims</code> and
<code>howmany_dims</code> are arrays of <code>fftw_iodim</code>.  Each <code>n</code> field must
be positive for <code>dims</code> and nonnegative for <code>howmany_dims</code>, while both
<code>rank</code> and <code>howmany_rank</code> must be nonnegative.
</p>
<p>For example, the <code>howmany</code> parameter in the advanced complex-DFT
interface corresponds to <code>howmany_rank</code> = 1,
<code>howmany_dims[0].n</code> = <code>howmany</code>, <code>howmany_dims[0].is</code> =
<code>idist</code>, and <code>howmany_dims[0].os</code> = <code>odist</code>.
<a name="index-howmany-loop"></a>
<a name="index-dist-1"></a>
(To compute a single transform, you can just use <code>howmany_rank</code> = 0.)
</p>

<p>A row-major multidimensional array with dimensions <code>n[rank]</code>
(see <a href="Row_002dmajor-Format.html#Row_002dmajor-Format">Row-major Format</a>) corresponds to <code>dims[i].n</code> =
<code>n[i]</code> and the recurrence <code>dims[i].is</code> = <code>n[i+1] *
dims[i+1].is</code> (similarly for <code>os</code>).  The stride of the last
(<code>i=rank-1</code>) dimension is the overall stride of the array.
e.g. to be equivalent to the advanced complex-DFT interface, you would
have <code>dims[rank-1].is</code> = <code>istride</code> and
<code>dims[rank-1].os</code> = <code>ostride</code>.
<a name="index-row_002dmajor-3"></a>
</p>

<p>In general, we only guarantee FFTW to return a non-<code>NULL</code> plan if
the vector and transform dimensions correspond to a set of distinct
indices, and for in-place transforms the input/output strides should
be the same.
</p>
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